Sara buys soil from a gardener to fill a planter. the gardener has 1.5 cubic feet of soil to sell for $6. sara buys just enough soil to fill her planter, which is a prism with a height of 0.5 feet and a square base where each side length is 1 foot. how much will it cost sara to completely fill her planter with soil?

The first thing you should do is calculate the volume of the prism.
 We have then:
 V = h * L ^ 2
 Where,
 L: side of the square base.
 h: height.
 Substituting we have:
 V = (0.5) * (1) ^ 2 = 0.5 feet ^ 3.
 Then, we can make the following rule of three:
 1.5 feet ^ 3 ---> 6
 0.5 feet ^ 3 ---> x
 Clearing x we have:
 x = (0.5 / 1.5) * (6) = 2 $
 
 it will cost Sara 2 $ to completely fill her planter with soil

Subtracting is the opposite of adding